1. Field of the Invention
The invention relates to operational amplifiers, and, more particularly, to operational amplifiers for low supply voltage applications, which employ unique DC biasing and current mirroring schemes.
2. Background of the Invention
Operational amplifiers are very high gain DC-coupled amplifiers with single-ended output. Typically, differential amplifiers are used as the input stage for operational amplifiers (sometimes referred to herein as "op-amp"). Operational amplifiers may be thought of as devices with two inputs and one output. The two inputs may be thought of as "non-inverting" and "inverting" signals. When the non-inverting input goes more positive than the inverting input, the output of the amplifier goes positive, and vice-versa. Operational amplifiers have enormous voltage gain and they are typically used with feedback.
An ideal op-amp with external feedback has a relatively simple behavior. First, the output from the op-amp attempts to do whatever is necessary to make the voltage difference between the op-amp inputs equal 0. Second, the op-amp draws very little input current which, in the ideal situation, approximates no current. In effect, operational amplifiers behave by "looking" at the input terminals and swinging the output terminal around so that the external feedback network brings the input differential to 0 (if possible). In this ideal op-amp, the output voltage swing may be as great as the range of power supply operating voltage. For example, if a power supply yielding a 0 to 5 volt supply voltage range operates an op-amp, then the output voltage swing can theoretically be 0-5 volts. Likewise, in the case of a power supply yielding a 0 to 2.7 volt supply voltage range for an op-amp, a 0-2.7 volt output swing is theoretically possible. So these are the ideal cases for a 0-5 supply voltage and a 0-2.7 supply voltage, respectively.
In the real world, these ideal conditions cannot be absolutely met. The real world op-amp will be operable around some range at about the mid-voltage of the power supply operating voltage range, and the output voltage will swing around this mid-voltage plus or minus some .DELTA.V, which is something less than the theoretical output voltage possible given the range of supply voltage available from the power supply. In practice, op-amps typically work best at around a mid voltage, which is considered the common level voltage. That common level voltage will then swing up to the plus or minus .DELTA.V which is possible based on the particular configuration.
One technique to obtain desired operation of an amplifier, such as desired output voltage swing, is termed "biasing." In biasing techniques, voltages are applied to corresponding transistor elements through biasing circuitry networks. These biasing circuitry networks can serve a number of purposes, and, in any case of use of a biasing network, biasing for one purpose may affect other aspects of amplifier circuit operation.
One biasing technique, sometimes referred to as matched base-emitter biasing, can be used to make what is called a current mirror. A current mirror is important in differential amplifier devices in a number of instances where a current source is needed. A common arrangement for a current mirror is sometimes referred to as a Wilson mirror. The Wilson mirror is a pair of gm p-channel or n-channel devices connected in series. This arrangement has one leg carrying a reference current which is mirrored by the two transistors in series in the other leg. This technique works to sufficiently mirror current in most cases. However, in low power applications, a relatively significant voltage drop occurs across the two transistors in series. That voltage drop takes up a significant amount of the possible output voltage swing of the amplifier, so the actual output voltage swing is often within a narrower range than the range of operating voltage available from the power supply.
The present invention is an operational amplifier designed to work with low supply voltages and yet meet the output range performance requirements found in higher voltage applications. The present invention amplifier is thought to be particularly effective when employed in battery powered or portable communication products. In particular, the amplifier solves many diverse problems found in low power applications in the related art, for example, problems of gain-band width, output swing, stability, and manufacturability. As will become apparent, this amplifier resolves many of these problems of the prior art due to the amplifier's unique DC biasing scheme for sinking and sourcing transistors and its unique current mirroring scheme to form the single-ended output.